One of the best kept secrets of the war involved the Japanese balloon bomb offensive, prompted by the Doolittle raid on Tokyo on April 18, 1942, as a means of direct reprisal against the U.S. mainland.

Some 9,000 balloons made of paper or rubberized silk and carrying anti-personnel and incendiary bombs were launched from Japan during a five-month period, to be carried by high altitude winds more than 6,000 miles eastward across the Pacific to North America. Perhaps a thousand of these reached this continent, but there were only about 285 reported incidents.